It is well known that the speed of sound in any medium is determined by the density of the medium and its compressibility. Sound tends to travel at high speed in a medium that is either relatively incompressible, or has a low density. It is known that sound travels at a relatively high speed of about 1500 m/s in water, whilst all the water is very dense, it is also highly incompressible.
It is also well known that sound travels at low speed in water containing a small proportion of bubbles. Water containing a small percentage of bubbles is still dense, but due to the compressible gas in the bubbles, it is relatively compressible. Consequently, the speed of sound in bubbly media may drop to 50 m/s or less.
Hitherto there have been arrangements which can use the compliant properties of a sheet of bubbles of a gas in water to funnel sound. Such an arrangement can form “walls” of sound which are suitable, for instance, for guiding fish away from water offtakes where they might be entrained and killed. The sound maybe generated by a pneumatic source, or by an underwater loudspeaker or “projector”.
The difference in speeds of sound in water with or without bubbles may be used to cause channelling of sound. In a bubble plume in water, the sound within the plume will travel relatively slowly, and that outside relatively fast, so that the wave front becomes “bent” towards the axis of the plume. In other words, the sound is trapped within the plume. This technology has been shown to be effective in applications where the sound is used to prevent fish kill by guiding fish away from hazardous areas, such as water offtakes.
Existing technology has four main disadvantages. First, it requires the use of a compressor to generate the bubbles. Frequently, where the line along which the fish must be guided is long, a long line of bubbles is required, which may require the use of large compressors. Often, the fish must be deterred from entering an area at all times, so that the compressor must be operated non-stop. This may cost a great deal as a result of the electricity used. In addition, where the water is deep, and a bubble curtain to the sea or river bed is required (that is, is has to exclude fish at all depths) the compressible gas must be pumped in to form the bubble curtain at the bottom of the water channel. Since the ambient pressure at that depth will be greater than atmospheric pressure, in order to retain a given volumetric flow at that depth, a greater volume of compressible gas at atmospheric pressure has to be pumped. For instance, at a depth of only 10 meters, the compressible gas flow requirements are roughly doubled. Clearly, this will further increase the cost of providing the compressed compressible gas.
Second, it is difficult to ensure that the line of bubbles is even. For instance, it is often the case that the wall of bubbles will be formed by means of a long pipe or pipe drilled with small holes at regular intervals, laid along the sea or river bed. Compressed compressible gas is forced into this pipe, and is released via the small holes. The flow of compressible gas through any one of the holes is controlled by the differential pressure between the inside of the pipe and the ambient pressure in the water outside the pipe. The greater this differential pressure, the greater the flow of compressible gas that will occur. If the ambient pressure at one region of the pipe is different to that at another point, the compressible gas will tend to selectively flow through the holes where the ambient pressure is the lowest. Thus, the bubble pipes tend to selectively bubble at the points in the pipe which are the highest in the water. To some extent, this effect may be minimised by making the holes small, and using a high internal pressure within the pipe (that is, of the compressed compressible gas), such that the differential pressure differences due to the ambient pressure differences are small when compared with the overall differential pressure. However, this means that the compressible gas must be at an even greater pressure than the ambient pressure, and the volumetric flow required from the compressor is even higher as a result.
Thirdly, where this approach is used, if the flow of compressible gas is to be reasonably low irrespective of the high internal pressure, the holes must often be very small if the overall flow of compressible gas for the entire bubble curtain is to be sufficiently low. Under these circumstances, it is easy for the holes to block as a result of small particles of debris carried by the compressed compressible gas, or by corrosion, or by biofouling.
In addition, the volume of compressible gas as a function of the depth of the bubble sheet is not under the control of the user. As a consequence of the diminishing ambient pressure they experience, the bubbles released from the pipe will expand as they rise through the water, such that the volume of compressible gas in the water will vary as the bubble sheet ascends towards the water surface. Thus, the acoustical behaviour of the bubbles will change with depth, which makes an optimal design of such a system difficult or impossible to achieve.
This is also made more difficult by the fact that the bubbles have significant buoyancy, and tend to entrain water and carry it to the surface. This may be seen by the typical appearance of an “upwelling” of water with the bubbles at the water surface. The fact that the water is entrained and helps to carry the bubbles to the surface means that the time that the bubbles spend in the water is reduced, that is, a larger volume of bubbles has to be provided in order to ensure a given volume of compressible gas in the water, than would be the case if there were no entrainment. This, again, means that the volume of compressed compressible gas required is further increased.
Finally, whereas the bubbles may initially create a laminar “wall”, as they ascend through the water a point is reached where the bubble sheet breaks up into swirls and patches of bubbles. This is thought to be a consequence of the sheet becoming turbulent as a result of the buoyant ascent. A similar phenomenon may be observed in the smoke and flame above a candle. Initially, the flame from a candle rises upwards in a linear and well-ordered manner. However, an inch or two above the candle, the smoke undergoes a transition and breaks up into swirls and eddies of smoke. This effect is thought to be very similar physically to the effect seen in the bubble sheet, where typically having risen a few meters as a linear sheet the bubbles break up into large eddies. Since there may be large spaces in between the eddies, their ability to carry sound is significantly decreased. As the ascent of the bubbles continues, they may arrive at the surface in periodic “gouts” of compressible gas.